The prior art of the present invention comprises documents FR-A-1,557,934 and EP-A1-1,541,834.
The present applicant has undertaken work pertaining to a propulsion unit architecture with two or more than two distributed fans. The purposes of the latter are to seek to optimize the propulsion performance owing to a by-pass ratio, while retaining an acceptable ground clearance of the aircraft and smaller fans, with different fan ratings.
The block diagram of such a unit is shown in FIGS. 1 and 2. A gas generator 3 traditionally comprises a compression unit supplying air to an annular compression chamber; the combustion gases leaving the chamber drive one or several turbines, mechanically connected to the compressor, and here a free turbine 5. The latter is secured to a power shaft 6 coaxial to the gas generator 3. This power shaft 6 drives, through appropriate conical gears, two intermediate radial shafts 8 and 8′ in particular arranged at a right angle relative to the axis of the power shaft 6. The intermediate radial shafts each drive a fan shaft 9, 9′ offset relative to the axis of the power shaft, i.e., having an offset axis and being parallel to the axis. The power transmission is done via first gears 2 and 2′ with bevel pinions between the shaft 6 and the radial shafts 8 and 8′ and second gears 4 and 4′ with bevel pinions between the radial shafts 8, 8′ and the fan shafts 9 and 9′. The shafts of the fans 9 and 9′ each drive a fan 10, 10′ with an axis parallel to that of the motor. Such an arrangement makes it possible to achieve the aims set out above.
According to the prior art known by the applicant, the solution to the transmission problem between the free turbine and the fan shaft consists of using, for each of the fans, angular members with single bevel gears at 45°: A first gear, with two wheels of the same diameter, one on the shaft of the free turbine and the other on the radial shaft, and a second tapered gear at 45° with two wheels of different diameters connecting the radial shaft to that of the fan.
One can see that the two fans are inevitably called upon to operate at very slightly different ratings, even in a normal situation. Indeed, during nominal operation, a slight asymmetry necessarily appears between the torques consumed in the two fans for example resulting from a different angle of attack of the air flows between the two fans, different frictions in the bearings, slightly different inertias and blading shapes.
Yet the kinematic diagram proposed above is based on a single angular member between the turbine shaft and two radial shafts making it possible to provide the required power to the modules of the fans. As a result, it is kinematically impossible for the two lines of fan shafts to rotate at different speeds. Such an arrangement results in driving the generation of stray shearing stresses, which may potentially have a very high amplitude, in the shaft lines of the fans.
The invention relates to the development of means able to provide a non-uniform kinematic distribution capacity between the two fans.
The invention also aims to propose a propulsion system architecture with multiple distributed fans equipped with a separating device making it possible to separate the kinematic behaviors of the transmission shafts of the fans and thus to be able to introduce differential loads on each of them without constraining the entire transmission chain.